A portable terminal apparatus such as a smart phone has a secondary battery which is chargeable and built thereinto. A charging AC adaptor is connected to the portable terminal apparatus in a wired manner, and charges the secondary battery. In recent years, a noncontact charging function has been installed in a portable terminal apparatus. The portable terminal apparatus includes a power reception coil receiving power, a power reception circuit generating power through the power reception coil, a charging circuit charging a secondary battery, and the like, and thus realizes a noncontact charging function. The noncontact charging function applies noncontact power transmission in which power is transmitted from a power transmission coil in a noncontact manner, and the power is received by a power reception coil.
Regarding the noncontact power transmission, a method is widely used in which power is transmitted through electromagnetic induction between a power transmission coil provided in a power transmission device and a power reception coil provided in a portable terminal apparatus. A frequency bandwidth used for electromagnetic induction is about 100 kHz to 200 kHz. A charging stand has a noncontact power transmission function. A surface of the portable terminal apparatus is planar, and thus a surface of the charging stand, which is a power transmission device on which the portable terminal apparatus, is placed is also planar. If the portable terminal apparatus is placed at any position on the surface of the charging stand, the charging stand detects a position of the portable terminal apparatus, moves the power transmission coil such that the power transmission coil and the power reception coil have an optimal positional relationship, and then charges the portable terminal apparatus. Fine position adjustment is further performed during charging, and thus power transmission efficiency is increased.
The use of a noncontact charging apparatus is not limited to a portable terminal apparatus such as a smart phone having a thin shape. Even in a portable terminal apparatus or an electronic instrument having some extent of thickness, and further having a protrusion, a secondary battery built into the portable terminal apparatus or the electronic instrument can be charged by using the noncontact charging apparatus. For example, a noncontact charging apparatus is used in a box-shaped portable electronic instrument such as a portable printer or a portable video camera, or a toy. A charging stand collectively charges a plurality of such instruments.
In a case where power is sent to an electronic instrument through noncontact power transmission, noise tends to be generated in a power transmission device or a power reception device. In order to suppress noise generated in the power transmission device and the power reception device, the power transmission device and the power reception device are surrounded by a metallic box. In order to increase power transmission efficiency, a power transmission coil built into the power transmission device and a power reception coil built into the power reception device are required to be maintained at positions separated from each other by a predetermined distance. In a case where the power transmission device and the power reception device are surrounded by the metallic box, it is hard to check whether or not the devices are held to be separated from each other by a predetermined distance from the outside of the box.